When I first got into Ponies again, I went hunting for a message board to run into more pony people. I looked at both boards and decided to join the Trading Post. I was intimidated by the bigger Arena so I started here.
I was exclusively a TP member for over a year. I enjoyed how quiet and low key everything was. The layout of this board is really nice, it's easy to find everything and use all the features. Everyone here is very friendly and lovely to talk to.
I decided to join the Arena as well a few months later. I won't lie, it was totally to gain access to some pony sales I had my eye on, but I tried to be active in the rest of the board as well. I'm currently involved in my first swap on the Arena. Not that I was trying to avoid them on the TP, but it just so happened that I found a swap on the Arena that really sparked my interest.
I enjoy both boards for different reasons. The Arena has an excited energy. Everything is constantly moving, there are tons of people posting everywhere. They get most of the new pony fans from FIM it seems. So we have lots of young girls excited about ponies for the first time and having a great time of it.
At the same time, the Trading Post seems to have a better sense of community for me. I keep seeing the same people and get a sense of who some of them are and which ponies are their favorites. It also seems we are mostly the G1 crowd here. On the Arena it's very rare when I join in on a thread with people I've seen before. There's just too many running around. I also enjoy the layout of the Trading Post better. My absolute favorite thing the TP does is stay on the same thread you were looking at when you log in. Any time you log in at the Arena, it takes you to the main page and you have to find the thread you were reading again. So annoying!
So I think each board has their strengths and weaknesses. But each also offers some of the same things: people who love ponies and want to enjoy them with others who love the same. I send lots of pony love to each board. They are both great communities to be a part of.